In recent years, increased production of biofuel from edible grain (for example, corn, potato, and sugar cane) has led to high food prices, and it is thus urgently necessary to produce ethanol from non-edible carbon source soft biomass (for example, rice straw, wheat straw, bagasse, rice husk, cotton, bamboo, paper, corn stover, or other wastes).
It has been proposed that an acid treatment or a supercritical treatment is made on biomass containing cellulose or hemicellulose to prepare from the source material glucose which microorganisms can utilize for fermentation.
Conventional methods for producing glucose from a cellulose-based material include acid saccharification methods and enzyme saccharification methods. Regarding acid saccharification methods, it has been known a dilute acid saccharification method for saccharifying a cellulose-based material with a dilute acid at high temperatures (200° C. or greater) as well as a method of saccharifying a cellulose-based material with concentrated sulfuric acid or the like. However, in either method, a cellulose-based material is hydrolysed under severe conditions, which results in a degraded product, glucose, from the cellulose-based material and a subsequent secondary reaction of degrading the glucose. Thus, yield of saccharification is as poor as about 50%, and the product of the reaction of degrading the glucose needs to be removed from the saccharified solution. There are some problems in use of the saccharified solution without removal of the product of the reaction of degrading the glucose, as a carbon source for fermentation.
Meanwhile, the enzyme saccharification method allows the saccharification of a cellulosic material under mild conditions, which results in a lower rate of reaction for saccharification, and thus it is problematic in that it takes a longer period of time for sufficient saccharification. In addition, it is problematic also in higher enzyme costs since the method requires larger amounts of enzyme for sufficient saccharification due to the lower titer of commercially available enzymes for use in saccharification.
An attempt has been made to modify microorganisms which originally cannot utilize the principal components, such as cellulose and hemicellulose, of soft biomass for fermentation, using bioengineering techniques to attain direct ethanol fermentation from the non-edible carbon sources. As such bioengineering techniques, cell surface-displaying techniques are suitably used. For example, yeasts that display on the surface a group of cellulolytic enzymes have been created by cell surface-displaying techniques (Patent Documents 1 and 2). Although the yeast Saccharomyces cerevisiae cannot metabolize xylose, Saccharomyces cerevisiae that displays on the cell surface xylan-degrading enzymes xylanase 2 (XYNII) from Trichoderma reesei and β-xylosidase (XylA) from Aspergillus oryzae and that expresses a xylose reductase (XR) gene and a xylitol dehydrogenase (XDH) gene (both from Pichia stipitis) and a xylulokinase (XK) gene (from Saccharomyces cerevisiae) has been created, and attempted to produce ethanol from xylan in birch trees (Non-Patent Document 1).
However, further research is required in view of industrial usefulness. The biomass of cellulose has a crystalline portion and a noncrystalline portion. The enzymatic hydrolytic reaction is easier on the noncrystalline portion than on the crystalline portion. It is thought that the rate of hydrolysis is lower on the crystalline portion which has rigid intramolecular and intermolecular hydrogen bonds. It is important to more efficiently carry out the hydrolysis of cellulose which has such a complex configuration.